Juvenile emigration patterns are documented in mantled howling monkeys but the specific impetus is unknown. To determine if emigration is hormonally mediated, 193 fecal samples were collected from 25 juveniles in 5 social groups on Hacienda La Pacifica during June-August, 1993-1995. Samples were analyzed for estradiol, progesterone, and testosterone concentration. Mean concentrations for each individual for each year were determined, and were compared by age (1-1.9 yrs; 2-2.9 yrs.; 3-3.9 yrs), by sex (male, female), and by emigration outcome (remained, emigrated during year, emigrated during study). All one-year olds remained in natal groups, with no differences in hormone concentrations between males and females. Two-year old females that remained in groups had higher mean concentrations of all hormones than females that emigrated. This pattern reversed at three years of age Females remaining in groups had lower mean hormone concentrations than those emigrating. For two- and three-year old males, those that remained in groups had lower concentrations than those that emigrated. In contrast, for two- and three-year old males being forced out of the group during the study, all hormonal values were very